blk: bring MAP_HUGETLB-based buffer pool to KernelDevice.

blk: bring MAP_HUGETLB-based buffer pool to KernelDevice.

The idea here is to bring a pool of `mmap`-allocated,
constantly-sized buffers which would take precedence
over the 2 MB-aligned, THP-based mechanism. On first
attempt to acquire a 4 MB buffer, KernelDevice mmaps
`bdev_read_preallocated_huge_buffer_num` (default 128)
memory regions using the MAP_HUGETLB option. If this
fails, the entire process is aborted. Buffers, after
their life-times going over, are recycled with lock-
free queue shared across entire process.

Remember about allocating the appropriate number of
huge pages in the system! For instance:

```
echo 256 | sudo tee /proc/sys/vm/nr_hugepages
```

This commit bases on / cherry-picks with changes
897a4932bee5cba3641c18619cccd0ee945bfcf8.

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>